The invention concerns a control system for a paper or board machine, by means of which the transverse profile of properties of the web to be produced, in particular its thickness profile, is regulated. The control system, more particularly, comprises a number of actuators and a corresponding number of actuator controllers, said actuators being fitted to be effective across the width of the web whose profile is to be regulated, and said control system including a process computer of an equivalent logical component as well as a feedback branch provided with an arrangement for measurement of the web profile to be regulated.
As is known from prior art, a pulp suspension jet is fed out of the discharge opening in paper or board machines onto a forming wire or into the gap between forming wires. The transverse profile of the discharge opening of the headbox also determines the profile of the pulp jet. The profile of the discharge opening is regulated, and by means of this regulation it is also possible to compensate for any flaws in the pulp jet that have arisen in or before the headbox.
In the prior art, a control system for the grammage profile lip of the headbox of a paper machine is known, which system comprises an angle-gear/stepping motor control device, wherein the profile bar of the regulator lips is controlled by means of adjusting spindles attached to the profile bar at intervals of about 10 . . . 15 cm, said spindles being displaced by stepping motors attached to their opposite ends by the intermediate of angle gears. The prior art control systems for adjusting spindles involve a number of drawbacks, most of which arise from the fact that the control of each actuator takes place separately directly from the system placed at a higher level in the hierarchy, which results, e.g., in the following drawbacks:
the stepping motors are controlled in parallel, so that several wires are required for each individual actuator, because normally the number of actuators is about 50 to 100, PA1 as the actuators have no local intelligence, the control system placed higher in the hierarchy has to take care of the regulation of all of the actuators at the same time, which, of course, burdens said regulation system particularly heavily, PA1 owing to said burdening, the speed of the system also remains low and the response times become long, PA1 since all the actuators are regulated in the same way, local conditions and mechanical differences, e.g., in the angle gears, cannot be taken into account, so that the accuracy of regulation is deficient and detecting of erroneous operations is difficult.
As has come out above at least partly, by means of the prior art technology, the profiling of the profile bar of the discharge opening of the headbox takes place by separate control of each regulation gear by means of handling sequences carried out one after the other. In order that the positioning should be successful with the required accuracy of about 10 .mu.m, electronics that measure the locations of the adjusting spindles are also required. Typically, the positioning takes place for about 5 seconds per actuator, and the reading of measurement alone takes 1.2 seconds. Thus, running through all of the actuators on a profile bar with 80 adjusting spindles takes from 1.5 to 6.5 minutes. Information is obtained on the state of an actuator momentarily only.
As is known in the prior art, the systems of cables from the processor cabinet are arranged by means of two system cables and electricity supply cables from the dry spaces to the wiring cabinet placed at the headbox (distance 50 to 250 meters), from where separate candles are passed to the location detector and stepping motor of each actuator (10 to 30 meters).
In paper and board machines and in related size presses, coating devices and calenders, the profile of the web that is being produced is regulated by means of devices in which, at least partly, the same problems are present as in the above prior-art systems for the regulation of the profile of the discharge opening of the headbox. Thus, the present invention is not confined to a control system for the discharge opening of a headbox alone, but it can be applied in various regulation devices for paper and board machines as well as for their finishing equipment by means of which devices the transverse profile of properties of the web is controlled. In addition to the devices mentioned above, further prior-art devices that affect the transverse profile of the web are, for example, steam boxes fitted in the press section of a paper machine, devices for profiling of the coating blades in a coating machine, as well as electric induction heating devices fitted, e.g., in connection with calender rolls, by means of which heating devices the profile of linear loads in a calendering nip is controlled.
In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that the various actuators in the control system are provided with intelligent actuator controllers and that the data communications in the control hierarchy between a higher control device and the various actuator controllers are arranged along a bus common of the various actuator controllers.
In the control system in accordance with the invention, the regulation is based on decentralized intelligence of actuators, which is just parametered by the higher system by giving the set values. Each actuator controller is seen upwards as an independent unit, to which a set value is given in digital form along the serial bus, whereupon the actuator controller handles the mechanical regulation independently in accordance with its own measurement-regulation algorithm.
The controlling apparatus placed at a higher level in the control hierarchy may be a bus server specifically provided for this purpose, said server transferring set values from the automation system to the actuators and status and measurement values from the actuator towards the system. The automation system may also control the actuators directly based on a certain line discipline.
The bus between the automation system and the series of actuators may consist of a simple asynchronous line. Thus, one cable only is required to pass to the entire series of actuators. Moreover, operating electricity is needed for the control card and for the stepping motors.
Having transmitted the set values to the actuators, the system at the higher level is free from load; the regulation/measurement takes place in the actuators independently and simultaneously. Thus, the regulation does not load the system, and the speed of operation is substantially higher than in the prior art.
In the invention, each individual actuator that regulates the transverse profile of the web is arranged as an independent positioning device, which is, when necessary, capable of monitoring its own operation in real time. The actuators are connected to the system for the calculation of profiling by means of a serial bus, whereby the amount of operations taking place at the same time is limited by the data transfer speed or by the capacity of the power source only. In the invention, if a data transfer speed of, e.g., 19,200 bauds is used for the control of the 80 spindles in the headbox while the message length is 8+6 bytes, for every actuator a new set value is obtained in less than two seconds. If it is desired to carry out the invention so that only ten (10) actuators are operative at one time, the positioning of all of the actuators takes less than half a minute.
In the invention, the system of cables between the control system and the series of actuators can be accomplished by means of a comb-like voltage and data distribution chain, to which the actuator controllers themselves are connected in direct proximity of the actuator, which consists, e.g., of a gear, a position detector, and of a motor.
The electronic systems of the actuators can be accomplished as standard modules, which are identical in the case of each actuator. In connection with the installation, each device can be given its own code by means of rotating switches provided on a card, whereupon the device can operate correctly in the bus when it recognizes its own code. Thus, the higher controlling system transmits set values for all the actuators through the bus, and it can read the measurement value as well as the status, which contains status and error data.
The electronics card of each individual actuator has preferably its own processor, memory, serial-communication electronics, integrated control electronics for the stepping motor, and an A/D-converter for carrying out the measurement from a LVDT-detector. The software contained in the card handles the serial communications upwards, the control of the stepping motor on the basis of the set values given, and the feedback measurement regarding the completion of the adjustment. During running, if necessary, it is possible to examine, e.g., the operating temperature, the stiffness of the transmission system, the accuracy of the regulation, etc., quantities in whose values a substantial change causes an error message and discontinuation of the regulation. A quantity that is changed within normal limits is compensated for by, on the basis of an algorithm, calculating a suitable correction for the regulation or measurement.
The fact that the control and the diagnostics are based on software makes the device easily adaptable to different purposes of use and to different environments and even to control mechanics of a different type. Later fine tuning and improved algorithms can also be integrated in the device without difficulty.